1. Field of the Invention
The invention relates to a compressor impeller fastening for high speed turboengines according to the preamble of claim 1.
2. Discussion of Background
Compressor impellers of turboengines are connected to their driveshaft either nonpositively or positively. In the case of rising pressure ratios and consequently in the case of increasing operating torques and high circumferential speeds, positive torque transmission, that is to say a positive shaft/hub connection of the compressor impeller, is advantageous.
EP 0,522,630 B1 discloses a positive compressor impeller fastening which is produced by means of a multispline shaft. In this solution, the life of the shaft/hub connection is restricted due to the notches made by the female splines. Moreover, additional centering elements are necessary, which increase the costs of the compressor impeller. Owing to the production-related inaccuracies of multispline shafts, such a shaft/hub connection must always be balanced as a unit, the parts having to be marked accordingly for the purpose of identical reassembly. It is therefore not possible to use the compressor impeller with another shaft which is not balanced together with it. This, however, is a decisive disadvantage in the event of servicing.
Positive compressor impeller fastenings by means of a thread are known both from U.S. Pat. No. 3,961,867 and from WO 93/022778. Production-related inaccuracies of the thread are likewise a disadvantage in these cases. Moreover, the high operating torques occurring in compressor impellers necessitate high heightening and releasing torques. Particularly where larger compressor impellers are concerned, the releasing torques required for demounting are up to double the operating torque. Such forces can be exerted only by means of special tools or by means of a step-up gear. However, this markedly increases the outlay necessary for demounting compressor impellers. Another disadvantage of fastening the compressor impeller by means of a thread is that, when the compressor impeller is being mounted, the regions of the hub thread which first come into contact with the shaft thread have to cover a relatively long distance on the shaft thread until they reach their end position. Since the threads involved have scarcely any play, there is relatively high pressure between the individual thread parts, that is to say in a region without any lubrication. So-called scoring or deformation of the threads therefore occurs, so that different results are obtained during each new mounting operation. Such a connection consequently cannot be reproduced sufficiently. Moreover, these solutions relate to compressor impellers with a blind bore, which, as regards their shaft/hub connection, cannot be compared to compressor impellers which have a through bore.
According to "Informationen uber die Anwendung von Polygon-Verbindungen" [Information on the use of polygonal connections], of Fortuna-Werke Maschinenfabrik AG, Stuttgart-Bad Cannstatt, a spur pinion shaft and a compressor impeller for the blower of a cooling system are known. For the rotationally fixed connection of the compressor impeller on the shaft, the two components have a conical profile with a polygonal base area, the shaft cone being arranged on the shaft end. The shaft cone and hub cone, that is to say the actual connection point of the shaft and compressor impeller, are arranged on the compressor side of the rear wall of the compressor impeller and consequently at the mass center of gravity of the compressor impeller. When the turbocharger is in operation, this region of maximum stress concentration necessarily experiences the greatest expansion, so that the safety of the connection falls with a rising circumferential speed of the compressor impeller. High speed turboengines, such as, for example, turbochargers, reach circumferential speeds of 500 m/s and above. Circumferential speeds of this kind place substantially higher requirements on the torque transmission and on the safety of the shaft/hub connection. These requirements cannot be satisfied by the conventional state of the art.